1. Field of the Invention
This invention relates to processes for applying anti-corrosive coatings upon metallic objects, especially upon bottles for liquefied gas.
2. Description of the Prior Art
It is known that ferrous alloy objects such as gas bottles corrode very rapidly upon contact with the ambient atmosphere, particularly when they are exposed to the elements.
To protect them, one conventionally employs a process comprising three principal steps:
a shot or sand cleaning of the surface to be treated; PA1 a metallization referred to as "coating with zinc" on the cleaned surface; PA1 the application of at least one coat of a protective paint.
The shot cleaning operation simply seeks to mechanically clean the surface of the object to be protected with an abrasive in order to eliminate calamine and traces of rust which can be present thereon and to roughen this surface for the purpose of assisting in the adhesion of protective layers to the metal.
The metallization by zinc coating consists in projecting a molten or a zinc-aluminum alloy upon the surface to be protected. The thickness of the zinc layer is usually from 40 to 100 microns.
This metallization operation is relatively time consuming and expensive due to the nature of the metal employed, the energy expended to melt the metal and the time required for its projection. It has, therefore, already been proposed to substitute zinc metal with the application of primer paint containing a corrosion inhibiting pigment, but the corrosion resistance of a surface thus protected is notably less than that which the presence of zinc assures.
Finally, the deposition of an exterior coat of paint is customarily done twice. One starts by applying a primer coat of a thickness of 30 to 40 microns, the essential function of which is to seal the imperfections in the surface of the zinc layer which is very porous. The primer paint can be of various types: glycerophthalic, vinyl, styrenated-alkyl paints, etc., and it can be applied by pneumatic spray gun either electrostatically or by any other method. One thereafter applies upon this primer coat a finishing coat, whose purpose is to impart to the object to be treated its final appearance. There is generally used for this purpose a paint having a glycerophthalic resin and aminoplastic resin base, which is applied in the form of a coating of about 30 microns thickness and which is then dried by passage for about 30 minutes in an oven at a temperature on the order of 130.degree.-140.degree. C.
The aggregate of these operations known in the art is therefore time consuming and costly.